1. Field of the Invention
The present invention relates to adaptive modulation and coding methods and apparatus for use, for example, in wireless communication systems.
2. Description of the Related Art
FIG. 1 shows parts of a wireless communication system 1. The system includes a plurality of base stations 2, only one of which is shown in FIG. 1. The base station 2 serves a cell in which a plurality of individual users may be located. Each user has an individual user equipment (UE). Only the user equipments UE2, UE11 and UE50 are shown in FIG. 1. Each UE is, for example, a portable terminal (handset) or portable computer.
As is well known, in a code-division multiple access (CDMA) system the signals transmitted to different UEs from the base station (also known as “node B”) are distinguished by using different channelisation codes. In so-called third generation wireless communication systems a high speed downlink packet access (HSDPA) technique has been proposed for transmitting data in the downlink direction (from the base station to the UEs). In this technique a plurality of channels are available for transmitting the data. These channels have different channelisation codes. For example, there may be ten different channels C1 to C10 available for HSDPA in a given cell or sector of a cell. In HSDPA, downlink transmissions are divided up into a series of transmission time intervals (TTI) or frames, and a packet of data is transmitted on each different available channel to a selected UE. A new choice of which UE is served by which channel can be made in each TTI.
FIG. 2 shows an example of the operation of the HSDPA technique over a series of transmission time intervals TTI1 to TTI9. As shown in FIG. 2, in TTI1 it is determined that two packets will be sent to UE50, four packets will be sent to UE11 and four packets will be sent to UE2. Accordingly, two channels are allocated to UE50 and four channels each are allocated to UE11 and UE2. Thus, as shown in FIG. 1, UE50 is allocated channels C1 and C2, UE11 is allocated channels C3 to C6, and UE2 is allocated channels C7 to C10.
In the next transmission time interval TTI2 a new user equipment UE1 is sent one packet, and the remaining UEs specified in TTI1 continue to receive packets.
Thus, effectively the HSDPA system employs a number of parallel shared channels to transmit data in packet form from the base station to the different UEs. This system is expected to be used, for example, to support world wide web (WWW) browsing.
In the HSDPA system, channel state information (CSI) is made available to both the transmitter and the receiver, in order to realise a robust communication system structure. The HSDPA system is intended to increase the transmission rates and throughput, and to enhance the quality of service (QoS) experienced by different users. It transfers most of the functions from the base station controller (also known as the radio network controller or RNC) to the base transceiver station (node B).
The HSDPA system may also use a control technique referred to as an adaptive modulation and coding scheme (AMC) to enable the base station to select different modulation and/or coding schemes under different channel conditions.
The signal transmission quality for a channel between the transmitter and a receiver (UE) varies significantly over time. FIG. 3 shows an example of the variation of a signal-to-interference ratio (SIR) a downlink channel for four different users over a series of 5000 TTIs. This plot was obtained by a simulation. As illustrated, for a given UE the range of SIR values may be as much as from around +12 dB to −15 dB. The SIR value varies due to shadowing, Rayleigh fading, and change in distribution of the mobile UEs, as well as cellular area specifications including the propagation parameters and speeds of UEs.
FIG. 4 is a graph illustrating a relationship between a data transmission rate (throughput) and signal-to-interference ratio for four different modulation and coding combinations, also referred to as modulation-and-coding scheme (MCS) levels. The first three levels (MCS8, MCS6 and MCS5) are all quadrature amplitude modulation (QAM) schemes which differ from one another in the number (64 or 16) of constellation points used. The fourth level (MCS1) uses quadrature phase shift keying (QPSK) as its modulation scheme.
Each level uses coding defined by a coding parameter which, in this example, is expressed as a redundancy rate R. For the first two levels MCS8 and MCS6 the redundancy rate R is 3/4, and for the third and fourth levels MCS5 and MCS1 the redundancy rate is 1/2.
As can be seen from FIG. 4, for SIR values lower than around −4 dB MCS1 (QPSK, R=1/2) is the best available option. The characteristic of this level is plotted with circles in the figure.
For SIR values in the range from around −4 dB to around +2 dB, MCS5 (16QAM, R=1/2) provides the best transmission rate. The characteristic for this MCS level is illustrated by crosses in the figure.
For SIR values between around +2 dB and +8 dB MCS6 (16QAM, R=3/4) provides the best transmission rate. The characteristic for this MCS level is illustrated by diamond points in the figure.
Finally, for SIR values greater than around +8 dB, MCS8 (64 QAM, R=3/4) provides the best transmission rate. The characteristic of this combination is illustrated by square points in the figure.
In the HSDPA system a technique such as adaptive modulation and coding (AMC) is used to adapt the MCS level in accordance with the variations of the channel condition (e.g. SIR value).
According to the HSDPA standard (3GPP TS 25.214 V5.5.0 (2003-6)), each UE holds a channel quality indicator (CQI) mapping table. An example of the mapping table is shown in FIG. 5. As the table shows, for each CQI value various parameters are defined including a transport block size, a number of codes, a modulation type, and a reference power adjustment Δ. The transport block size represents a maximum number of bits which can be received in one TTI. The number of codes is the number of channelisation codes which are sent simultaneously to a single user within one TTI. The modulation type represents the type of modulation scheme, eg QPSK or 16QAM. The reference power adjustment Δ is a reduction to be applied to the transmitted power if the transmitted power is greater than that necessary for the signal to be receivable at the CQI value.
Each UE produces a measure of the quality of a downlink channel from the base station to the UE. Based on this measure and on the CQI mapping table the UE reports the highest CQI value for which a signal having the transport block size, number of codes and modulation for that value is receivable with a transport block error probability (also referred to as a Packet Error Rate (PER)) below a certain target value.
There may be a one-to-one correspondence between the CQI values and MCS levels, so that if desired the base station may directly take the reported CQI value as the MCS level to apply. For example, in one proposal (3GPP TSGR1-02-0459, “HSDPA CQI proposal”, 9-12 Apr. 2002, Paris, France), there are CQI values 1 to 30 which are intended to provide approximately a 1 dB step size between adjacent MCS levels at 10% PER. Alternatively, the base station may employ the reported CQI value for each UE, as well as information relating to the system limitations and available MCS levels, to identify the most efficient MCS level for the particular UE.
Thus, based on the reported CQI values, UEs that have better channels or are located in the vicinity of the base station can employ higher levels of MCS and therefore enjoy higher transmission rates. Effectively, the result is a classification of the transmission rates based on the channel quality of each UE.
Ideally, each UE reports a CQI value in every TTI and the base station is capable of setting a new MCS level for each available channel in every TTI.
The HSDPA system may also employ a hybrid automatic repeat request (H-ARQ) technique.
FIG. 6 is a schematic diagram for use in explaining how the H-ARQ technique works. In this example, the technique is a so-called stop-and-wait (SAW) version of the technique. The figure shows packet transmissions in a single downlink channel HSPDSCH1 over a series of successive TTIs, TTI1 to TTI9. In TTI2 a first packet is transmitted to UE1. Upon receiving a packet, each UE checks whether the transmission was error-free. If so, the UE sends an acknowledge message ACK back to the base station using an uplink control channel such as the dedicated physical control channel (DPCCH). If there was an error in the transmission of the received packet, the UE sends a non-acknowledge message NACK back to the base station using the uplink channel.
In the example shown in FIG. 6, the first packet transmitted to UE1 in TTI2 fails to be received error-free, and accordingly some time later, in TTI4, UE1 sends the NACK message to the base station. In the H-ARQ technique it is permitted for the next packet destined for a particular UE to be transmitted without waiting for the acknowledge or non-acknowledge message of a packet previously transmitted to the same UE. Thus, none of the transmission timeslots can go idle in the case of error-free channels, which gives the ability to schedule UEs freely. System capacity is saved while the overall performance of the system in terms of delivered data is improved.
For example, as shown in FIG. 6, before the NACK message for the first packet of UE1 is received by the base station, the base station transmits a second packet to UE1 in TTI4. Thus, this second packet for UE1 is transmitted before the first packet for UE1 is retransmitted in TTI7 in response to the NACK message for the first transmission of the first packet.
In the H-ARQ technique, an erroneously-received packet (failed packet) is subject to a so-called chase combining process. In this process a failed packet is resent by the transmitter and subsequently the receiver “soft” combines (for example using maximal ratio combining) all received copies of the same packet. The final SIR is determined as the sum of the respective SIRs of the two packets being combined. Thus, the chase combining process improves the SIR of the transmitted packets.
Further information regarding AMC and HARQ techniques may be found in 3GPP TR 25.848 V 4.0.0 (2001-03), Third Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Layer Aspects of UTRA High Speed Downlink Packet Access (release 4), March 2001, the entire content of which is incorporated herein by reference.
The switching between different MCS levels has been recognised as a very critical task, and recently there have been various proposals for optimising this switching. For example, in TSG R1-1-0589, TSG-RAN Working Group 1 meeting no. 20, Busan, Korea, May 21 to 25, 2001, NEC and Telecom MODUS jointly proposed an AMC technique in which the thresholds for switching between different MCS levels are adjusted based on the ACK/NACK signalling from the UE. If NACK is signalled, the base station increases the thresholds by an upward amount S1. If ACK is signalled, the base station decreases the thresholds by a downward amount S2. The adjustments to the thresholds are limited and, for simplicity, the differences between thresholds may be fixed. The ratio between the upward amount S1 and the downward amount S2 may be determined based on the target error rate.
This AMC method adjusts the thresholds between MCS levels to try to take into account different operating conditions in the wireless communication system. In particular, the optimum MCS levels under any particular signal conditions depend on the Doppler frequency (i.e. the speed at which the UE is moving) and the multi-path propagation conditions. For example, FIG. 7 shows the effect of the UE speed on the throughput-vs.-SIR characteristic for each of the different MCS levels in FIG. 4. Three lines are plotted per MCS level: the highest line corresponds to a low UE speed of 3 km/h (Doppler frequency Fd=5.555 Hz), the middle line corresponds to a medium UE speed of 60 km/h (Fd=111.112 Hz), and the lowest line corresponds to a high UE speed of 120 km/h (Fd=222.24 Hz). FIG. 7 shows that throughput declines as UE speed increases. It can also be seen that the optimum thresholds for switching between MCS levels are also changed as the UE speed changes.
FIG. 7 relates to a single-path Rayleigh fading mode. FIG. 8 shows the effect of different UE speeds under path conditions of two equal-gain paths. It can be seen that the characteristics are very different from FIG. 6, and it is clear that the optimum thresholds are changed as the path conditions change.
The method proposed by NEC/Telecom MODUS changes the thresholds as the operating conditions change but the method does not provide a satisfactory solution as it increases or decreases the threshold each time an ACK or NACK message is received, i.e. every frame. When the step size between thresholds for switching MCS levels is significant (eg a few dB) this appears to result in relatively poor performance at lower MCS levels for path conditions in which there is effectively a single dominant path, for example in open countryside.
In another AMC method proposed by NEC and Telecom MODUS in TSG R1-1-0589 the base station selects a MCS level based on the ACK/NACK signalling from the UE. For example, the base station lowers the MCS level if NACK is received, and increases the MCS level if ACK is received successively for a certain number of TTIs. This method has the advantage that it does not rely on results of measuring the channel quality to select the MCS levels. Thus, problems of measurement accuracy and reporting delay are avoided. However, this method appears to have relatively poor performance at high SIR values when there are two paths of comparable strength, for example in an urban environment.